A power transmission device that transfers engine power of a vehicle, such as an automobile, to a wheel has to transfer the power from the engine to the wheel. The device must tolerate radial or axial displacement and moment displacement from the wheel. The displacement occurs when the vehicle bounds or turns while running on a rough road surface. One end of a drive shaft provided between the engine side and the driving wheel side is coupled to a differential via a slidable constant velocity universal joint. The other end of the shaft is coupled to the wheel via a driving wheel bearing apparatus. The driving wheel bearing apparatus includes a fixed type constant velocity universal joint.
In recent years, the demand for enhanced fuel efficiency has grown sharply from the viewpoint of savings resources, environmental pollution, etc. Weight reduction of a wheel bearing apparatus, in particular, of the automobile parts has long received attention and strongly desired as a factor that fulfills such demands. Various proposals for a wheel bearing apparatus designed to achieve weight reduction have been made. It also becomes important to reduce costs by simplifying assembly and disassembly operations in an assembly site of the automobile or the like or a maintenance market.
A driving wheel bearing apparatus shown in FIG. 29 is a typical example that fulfils such demands. This driving wheel bearing apparatus has a structure where a double row rolling bearing 101 and a constant velocity universal joint 102 are detachably unitized. The double row rolling bearing 101 includes an outer member 103 with a body mounting flange 103b to be mounted on a car body. The body mounting flange 103b is integrally formed with the outer member 103. The outer member 103 has double row outer raceway surfaces 103a and 103a formed on its inner circumference. An inner member 106 includes a wheel hub 104 and an inner ring 105. The wheel hub 104 has a wheel mounting flange 104b for mounting a wheel (not shown). The wheel mounting flange 104b is integrally formed at one end of the wheel hub 104. The outer circumference of the wheel hub has an inner raceway surface 104a arranged opposite to one of the double row outer raceway surfaces 103a and 103a. A cylindrical portion 104c axially extends from the inner raceway surface 104a. The inner ring 105 is press-fit onto the cylindrical portion 104c of the wheel hub 104. The inner ring outer circumference includes an inner raceway surface 105a. The inner raceway surface 105a is arranged opposite to the other of the double row outer raceway surfaces 103a and 103a. Double row balls 108 and 108 are freely rollably contained between the raceway surfaces via a cage 107 placed between them. In addition, the inner ring 105 is axially secured to the wheel hub 104 by a caulked portion 109. The caulked portion 109 is formed by plastically deforming the end of the cylindrical portion 104c. Furthermore, a face spline 109a is formed at the end face of the caulked portion 109. The face spline 109a of the caulked portion 109 is formed while caulking is performed.
Seals 110 and 111 are mounted on the opening of an annular space formed between the outer member 103 and the inner member 106. The seals 110,111 prevent leakage of grease contained in the bearing and entry of rainwater, dust, etc. into the bearing from the outside.
The constant velocity universal joint 102 includes an outer joint member 112, a joint inner ring 113, a cage 114, and torque transmission balls 115. The outer joint member 112 has a cup-shaped mouth portion 116, a shoulder portion 117 that forms the bottom of the mouth portion 116, and a hollow shaft portion 118 that axially extends from the shoulder portion 117. The mouth portion 116, the shoulder portion 117, and the shaft portion 118 are formed integrally in the outer joint member 112. The inner circumference of the shaft portion 118 includes an internal thread 118a. A face spline 117a is formed at the end face of the shoulder portion 117. The face spline 117a engages the face spline 109a formed at the end face of the caulked portion 109. Rotational torque from a drive shaft (not shown) is transmitted to the wheel mounting flange 104b via the constant velocity universal joint 102 and the inner member 106.
A fastening bolt 119 is threadedly connected to the internal thread 118a of the shaft portion 118. The face splines 117a and 109a of the outer joint member 112 and the inner member 106, arranged opposite to each other, are supported with pressure by the fastening bolt 119. Thus, the double row rolling bearing 101 and the constant velocity universal joint 102 are detachably unitized. This makes it possible to realize a reduction in weight and size and simplify disassembly and assembly operations (refer to, for example, Patent Document JP-A-63-184501).
This driving wheel bearing apparatus enhances workability since the face spline 109a is formed while the caulked portion 109 is formed at the time of the oscillating caulking. Thus, this can reduce costs by reducing the number of processes. Since torque is transmitted by the face splines 109a and 117a, it is possible to realize a reduction in weight and size and simplify disassembly and assembly operations. However, since the face spline 109a is formed while the caulked portion 109 is formed at the time of the oscillating caulking, the surface of the tooth surface keeps the surface hardness obtained after cold plastic deformation. Thus, the surface hardness only shows a slight increase compared to the surface hardness obtained after the wheel hub 104 is forged. Since the fatigue strength generally increases proportionately with the surface hardness, a significant increase in the fatigue strength of the face spline 109a cannot be expected.
High frequency induction quenching may be performed on the face splines 109a and 117a to increase the surface hardness. These increases wear resistance and fatigue strength. However, this is undesirable because doing so not only affects engagement between the face splines 109a and 117a, due to distortion by heat treatment, but also causes a reduction in toughness due to the high degree of hardness.